How to calculate storage volume
Storage volume in cubic feet is length × width × height in feet. A storage unit's gross volume is its floor area multiplied by the ceiling height — 100 square feet at 8 feet gives 800 cubic feet. Usable volume is lower: multiply gross volume by a packing efficiency of about 0.85, and subtract a further 15 per cent if you need an access aisle.
- L × W × H in feet
- 8 ft assumed ceiling
- 0.85 packing efficiency
The three volumes that get confused
| Term | Definition | Example: 10x10 unit |
|---|---|---|
| Floor area | Width × length, in square feet (sq ft) | 100 sq ft |
| Gross volume | Floor area × ceiling height, in cubic feet (cu ft) | 800 cu ft |
| Usable volume | Gross volume × packing efficiency | 680 cu ft at 0.85 |
| Usable with aisle | Usable volume × 0.85 | 578 cu ft |
Quoting floor area as if it were capacity is the most common error in storage sizing. You store by volume.
Calculating item volume
For a rectangular item, volume in cubic feet is length × width × height with every dimension in feet. A dresser 5 ft wide, 1.5 ft deep and 3 ft tall is 5 × 1.5 × 3 = 22.5 cu ft.
If you measured in inches, divide the product by 1,728 (12 × 12 × 12). A 60 × 18 × 36 inch dresser is 38,880 ÷ 1,728 = 22.5 cu ft.
Bounding box versus packed volume
The formula gives a bounding box. What matters in a storage unit is packed volume — the space an item effectively consumes once loaded, including what its shape wastes. A recliner's bounding box understates it, because nothing stacks on a curved back. A dining table's overstates it, because it loads on its side against a wall.
Our published item values are packed volumes, the same basis professional movers use on a cube sheet. They are in the published item dataset.
Worked example
A one-bedroom apartment: sofa (50) + armchair (15) + coffee table (10) + TV and stand (25) + bookcase (20) + dining table (30) + four chairs (20) + queen bed set (60) + double dresser (40) + two nightstands (12) + desk and chair (30) + 30 assorted boxes (100) = 412 cu ft.
Divide by 0.85 for moderate packing: 412 ÷ 0.85 = 485 cu ft of gross unit volume needed. The smallest unit above that is a 5x15 at 600 cu ft, but its 5 ft width is awkward for a sofa — so the practical answer is a 10x10 at 800 cu ft, filling about 61% of its usable space.
That is the whole method. The size calculator automates it over a 110-item catalog and adds the door-clearance check.
Fill rate, and why 85 per cent matters
Fill rate is your item volume divided by the unit's usable volume. Below 60% you have bought room you did not need; above 90% you are relying on everything going exactly to plan.
| Fill rate | What it means |
|---|---|
| Under 60% | Over-sized. Consider the size below. |
| 60–90% | The target range. Room for the items you forgot. |
| 90–100% | Tight. Achievable with careful packing, no access. |
| Over 100% | Does not fit. Size up or reduce. |
Related questions
How many cubic feet is a 10x10 storage unit?
800 cubic feet: 10 ft × 10 ft × 8 ft ceiling. Usable capacity is closer to 600 cu ft after realistic packing.
How do I convert square feet to cubic feet?
Multiply the floor area by the ceiling height. 100 sq ft × 8 ft = 800 cu ft. They are not interchangeable units — one measures floor, the other measures space.
How many cubic feet is a sofa?
About 50 cu ft packed for a three-cushion sofa, 35 for a loveseat and 60 for a sleeper sofa. The mechanism in a sofa bed adds both volume and weight.